Method of doping semi-conductive materials



July 1, 1958 F. D. Rosi METHOD OF' DOPING SEMI-CONDUCTIVE MATERIALS Filed April 27, 1955 zal INVENTOR.

fix-0 R05/ United States Patent ice METHOD F DOPING SEMI-CONDUCTIVE MATERIALS Fred D. Rosi, Plainsboro, N. J., assignor to Radio Corporation of America', a corporation of Delaware I This invention relates generally to an improved methodfor alloying a relatively low boiling or sublimation point material with a relatively high melting pointV semiconductive material. More particularly, but not necessarily exclusively, the invention relates to the alloying of low boiling or low sublimation point conductivity-typedetermining impurities with high melting point molten semi-conductive materials such as germanium and silicon.

It is known to control the type of conductivity of semiconductive materials by introducing thereinto small amounts of certain foreign materials generallytermed impurities The introduction of such impurities into semi-conductive materials is usually referred to as doping The type of conductivity established in the semi-conductor isA dependent upon the electron configuration of the atoms of the impurity material and of the host crystal. Thus, a substance whose atoms are capable of giving up electrons to the atoms of a particular substance is termed a donor impurity, and since there is a surplus of electrons available to carry va current, the semi-conductorl so doped is deemed to be of n-type (negative) conductivity. On the other hand, a substance whose atoms are capable of borrowing'or accepting electrons is termed an acceptor impurity, and since-there is a shortage o f electrons in the crystal lattice available for current conduction, the semi-conductor so doped is deemed to be of p-type (positive) conductivity. Y Y y `The usual and most directly convenient mannerof introducing impurities into a semi-conductor is to add Ythe impurity into a melt of the semi-conductive material. Some semi-conductive materials have relatively high melting points. Silicon, for example, melts at 1420 C. Great ditliculty is therefore encountered in attemptingto dope such high melting point semi-conductors with lower boiling point impurities inasmuch as such impurities boiloi either priorrto introduction into the semi-conductor rnelt or before alloying with the molten semi-conductor.

This is especially true when the boiling point of the impurity is greatly lower than the semi-conductor melting point. Some impurities sublime rather than boil at relatively low temperatures and it is intended throughout -the instant application to include-sublimation temperature in the phrase boiling point. Typical impurities having boiling points sufficiently low'to cause ditiiculty in improved method of alloying an impurity material with Y a semi-conductive material having a melting point either 2,841,559 Patented July 1, 1958 about the same or higher than the boiling point of said impurity material.

Another object of this invention is to providelan improved method of doping a growing crystal of a semiconductive material such as germanium or silicon with an impurity having a boiling .point lower than the melting points of saidv semi-conductive materials without boilingolf the impurity.

These and other objects and advantages `are accomplished according to the invention by employing a relatively high boiling point compound one of the constituents of which is the relatively volatile impurity material with which'it is desired to dope a melt of a semi-conductive material such as germanium or silicon. The other con stituent of the compound is an element whose valence is the sameas that of the semi-conductive material. Due to its equivalency such an element will not ionize in the semi-conductive materialvand affect its conductivity. An example of such a compound for germanium or silicon is lead` selenide. VThis compound is either added as a solid or melted and then added to the molten semi-conductive material. The molten semi-conductive material is then re-frozen as either a single crystal or polycrystalline ingot. Since the boiling point of the compound is much higher than the boiling point of the impurity material uncombined and is also highery than the melting point of the semi-conductive material, the relatively low boiling point impurity can be introduced into the molten semi-conductive material as a constituent of a compound which will not boil-oit and become lost.

The invention will be described in greater detail by reference to the accompanying drawing wherein:

Figure 1 is a cross-sectional, elevational View of an elongated crucible being drawn through an induction type furnace; and

Figure 2 is a partially cross-sectional, elevational view of an apparatus for pulling a crystal from a melt.

.Referring tothe drawing, a semi-conductive material such as silicon can be doped with a volatile impurity such,

as selenium employing the apparatus shown in Figure l. An elongated boatlike crucible 6 of graphite is charged with a rod-.shaped piece of base material 2 such as silicon. If it is desired to produce a single crystal of silicon, thenV a seed crystal 4 of silicon is placed at one end of the crucible 6 close to but not in contact with the silicon charge. Between the seed crystal 4 and the silicon charge 2 a stoichiometric compound 7 of selenium and lead is placed. The lead selenide may be in any convenient form, such as pellets, for example. To produce a polycrystalline ingot of silicon the seed crystal 4 is The crucible is placed within a quartz tubular enclosure 11 and is gradually drawn at about 2 .5 inches per hour or less, for example, through a ring-shaped induction heating element 12 starting at the seed crystal end of the crucible. The crucible is supported within the enclosure 11 by a-ring-shaped member 5. At the start of the pullingvoperation, when the temperature reaches the melting point of the silicon, part of the seed 4 melts as well as 'all of the lead-selenide alloy 7, and the adjacent part of the silicon charge 2. As the crucible with its contents is drawn through the heating element 12, successive segments of the charge 2 are melted. The molten lead selenide dissociates in the molten silicon thus pro! viding a quantity of selenium atoms in the silicon melt.

By reason of the segregation coeiricient of selenium in silicon, which is dened as the ratio of the impurity (selenium) concentration in the solid side of the interface of a growing crystal to the concentration on the liquid side of the interface, some of the selenium atoms available in the melt will segregate to and freeze-out with the silicon. The segregation coeicient of selenium in silicon is not known. However, it is known that segregation of some of the selenium atoms in a silicon melt does occur and are thusable to" establish the crystal conductivity-type as the molten materials re-crystallize as an extension of the seedv crystal` lattice. lle-crystallization occurs as the crucible leaves the heat Zone` created by the heating element 12. Since the melting point of the compound lead selenide is l088 C., it will have a much higher boiling point, and, while this boiling point is not known precisely, it is higher than the melting point of the silicon. Therefore, none of the selenium in the compound will be lost by vaporization and` is available to contribute to the conductivity of the silicon single crystal being grown.

With such a compound, the conductivity ofthe silicon 1.5-2.5 ohm-cm. The conductivity of the single crystal produced from this charge is n-type.

In another example, semi-conductive siliconA is doped withv a relatively low boiling point impurity such as cesium employing the apparatus shown in Figure 2. lt should be understood, however, that the apparatusshown in Figure 1 could likewise be employed, if desired, in

the instant example. ln Figure 2, a pot-type crucible 18 which may be of silica, for example, is supported by a pedestal 9 of tire-brick or other heat-insulating material in a quartz container 7, and heated byI conventional means (not shown). The crucible is charged with silicon 2t), for example 50 grams, and a seed of single crystal silicon 23 attached to a withdrawing apparatus 24 isi touched onto the surface of the molten silicon in the crucible. As the seed 23 is slowly withdrawn, an elongated single crystal 22 is atached thereto.v crystal is withdrawn, at a rate of 0.5 cm. per hour or less, for example. A stoichiometric compound of cesium stannide (CszSn) is added to the silicon melt in either pellet or powder form where it melts and dissociates throughout the molten mass. Again, by reason of the segregation co-eiiicient of cesium in silicon, some of the cesium atoms in the melt segregate to and freeze-out as an extension of the seed crystal lattice and thus contribute to the conductivity of the silicon single -crystal being grown. (1400 C.) which is about the same as that of silicon, its boiling point will be still higher. Therefore, none of the cesium carried in the compound will be lost by vaporization and is available to establish the conductivity of the silicon single crystal being grown.

lith a compound of cesium and tin, the conductivity of the silicon is affected only by the cesium atoms entering the single crystal lattice of silicon. The tin, being tetravalent, does not ionize in the silicon lattice and contribute conduction electrons or holes.

When doping tetravalent semi-conductors' according to the invention, a binary compound of the doping element and an element which is tetravalent is selected. As'has been indicated, the reason for choosing a tetravalent element is to insure that the conductivity of the grown crystal will be determined only by the impurity element itself. That is, since the compound dissociates in the molten semi-conductor, the conductivity of`the crystal might be affected by both constituents of the-compound if one were not tetravalent.

The following binary compounds maybe employed to dope tetravalent semi-conductors such as silicon. and germanium. For doping with selenium, PbSe has the requisite melting and boiling point (melting at 1088 C.). For doping with tellurium, SnTe (melting at 917 The seed Since the cesium stannide has a melting point C.) may be employed; and for doping with magnesium,

Mg2Sn (melting at 778 C.). Cesium may be introduced by the compound CsZSn (melting at 1400 C.). Lithium may be introduced by the compounds LiqSn (melting at 783 C.) or LiqPbZ (melting at 726C.). Calcium likewise may be introduced by the following compounds: CaSi (melting at 1245 C.), Ca2Pb (melting at 1110 C.), and CaTiz (melting at 14.50 C.).

It is to be noted that only melting points of the enumerated compounds have been given. This is because the precise boiling points are not known. Itis, however, certain that the boiling points of these compounds will be sufficiently higher than their melting points so that the compounds may be introduced in the various molten semi-conductors without their boiling cfr.

What is claimed is:

1. The method of alloying a conductivity-type-determiningmaterial with a semi-conductive element selected from the class consisting of germanium and silicon and having a particular valence, the melting point of said semiconductive element being higher than the boiling point of said conductivity-type-determining material, comprising the step of: adding into a meltV of said semi-conductive element a compound consisting of said conductivitytype-determining material and an elementy having a valence the same as that of said semi-conductive element, the boiling point of said compound being higher than the melting point of said semi-conductive element.

2-. The method of alloying a conductivity-type-determining impurity material selected from the class consising of selenium, magnesium, lithium, cesium, calcium, andV tellurium with a semi-conductive material selected from the class consisting of germanium and silicon, the melting point of said semi-conductive material being higher than the boiling point of saidy impurity material, comprising the step of: adding into a melt of said semiconductive material a binary compound of a tetravalent element and said impuritiy material, the boiling point of saidV compound being higher than the melting point of said semi-conductive material.

3. The method of alloying a conductivity-type-determining impurity material with a semi-conductive material selected from the class consisting of germanium and silicon, the melting point of said semi-conductive material being higher than the boiling point of said impurity material, comprising the step of: adding into a melt of said semi-conductive material a compoundl selected from the class consisting of: PbSe, Mg2Sn, Li7Pb2, Li7Sn, CszSn, CaSi, CaZPb, SnTe, PbTe, and CaTiZ.

4. The method of growing a single crystal of' a semiconductive element selected from the class consisting of germanium and silicon and having a particular valence and containing a conductivity-type-determining impurity material whose boiling point is lower than the melting point of said semi-conductive element comprising the steps oft: preparing a melt of said semi-conductive element and a compound consisting essentially of said impurity material and an element having a valence the same as that of said semi-conductive element, said compoundlhavin'g a boilingV point higher than the melting point of said semi-conductive element, and initiating and maintainingsingle crystalline growth of said semi-conductive element from said melt.

5. The method of growing a single crystal of a semiconductive element selected from the class consisting'l of germanium and silicon and having a particular valence and containing a conductivity-type-determining impurity material whose boiling point is lower than-the melting point of said semi-conductive element comprising the steps of: preparing a melt or said semi-conductiverelement and a compound consisting. essentially of said impurity material and an element having-a valence the same as tha-t of said semi-conductive element, said compound having a boiling point higher than the melting point of said semi-conductive element, contacting a seed crystal of said semi-conductive element to said melt until a single crystal of said semi-conductive element starts to grow attached thereto, and relatively moving said seed crystal away from said melt as said single crystal continues to grow.

6. The method of growing a single crystal of semiconductive element selected from the class consisting of' germanium and silicon and having a particular valence and containing a conductivity-type-determining impurity material whose boiling point is lower than the melting point of said semi-conductive element by horizontally zonemelting an elongated body of said semi-conductive element comprising the steps of: placing a mass of a compound consisting essentially of said impurity material and an element having a valence the same as that of said semi-conductive element at one end of said elongated body of said semi-conductive element, said compound having a boiling point higher than the melting point of said semiconductive element, melting said compound mass and a portion of said end of said elongated body of semi-conn ductive element, imitating single crystalline growth of said semi-conductive element from said melt, and thereafter melting and freezing-out successive adjacent portions of said elongated body of said semi-conductive element whereby a single crystal of said semi-conductive element is grown containing said impurity material.

7. The method according to claim 6 wherein said single crystalline growth is initiated by contacting a seed crystal of said semi-conductive element to said melt.

8. The method of growing a single crystal of silicon containing impurity centers of selenium comprising the steps of: preparing a melt of silicon and lead selenide, and initiating and maintaining single crystalline growth of said Silicon from said melt.

9. The method according to claim 8 wherein said single crystalline growth is initiated by contacting a seed crystal of silicon to said melt, and thereafter maintaining said References Cited inthe le of this patent UNITED STATES PATENTS 2,530,110 Woodyard Nov. 14, 1950 

1. THE METHOD OF ALLOYING A CONDUCTIVITY-TYPE-DETERMINING MATERIAL WITH A SEMI-CONDUCTIVE ELEMENT SELECTED FROM THE CLASS CONSISTING OF GERMANIUM AND SILICON AND HAVING A PARTICULAR VALENCE, THE MELTING POINT OF SAID SEMICONDUCTIVE ELEMENT BEING HIGHER THAN THE BOILING POINT OF SAID CONDUCTIVITY-TYPE-DETERMINING MATERIAL, COMPRISING THE STEP OF: ADDING INTO A MELT OF SAID SEMI-CONDUCTIVE ELEMENT A COMPOUND CONSISTING OF SAID CONDUCTIVITYTYPE-DETERMINING MATERIAL AND AN ELEMENT HAVING A VALENCE THE SAME AS THAT OF SAID SEMI-CONDUCTIVE ELEMENT, THE BOILING POINT OF SAID COMPOUND BEING HIGHER THAN THE MELTING POINT OF SAID SEMI-CONDUCTIVE ELEMENT. 